We are developing a novel kind of tissue spectrometer, which utilizes total scattered light to quantitatively and non-invasively measure the absorption and scattering coefficients in highly scattering media. The method is based on the use of a radio frequency-modulated light source. Measurement of phase shift and demodulation ratio are used to independently determine the scattering and absorption coefficients along the average path traveled by light in the tissue between this source and a detector. In our apparatus, the phase and demodulation can be accurately measured in real time on a millisecond time base. The important field of application of this tissue spectrometer is in medicine for measuring low concentrations of absorbing metabolites in tissues. In the proposed prototype, the modulated source consists of an array of laser diodes emitting at selected wavelengths relevant to the oxygenation state of the tissue. During Phase I of this project, a prototype instrument for research purposes will be assembled and tested both for integration of new components and limits of the technology. In Phase II absorption and scattering coefficients at different tissue locations will allow imaging of optical properties of tissues. Clinical tests will be performed at the end of Phase II.